1. Field of the Invention
The present invention relates to a photovoltaic device having high photovoltaic efficiency, that is one in which, the incident light is effectively converted into electrical energy and the generated electrical energy is conducted to the external device with reduced loss. The present invention also relates to a method for making the same.
2. Description of the Related Art
Various types of photovoltaic devices have been used as isolated power sources for electrical equipment and alternative energy sources for systematic electric power supply, and there has been continued research and development.
Photovoltaic elements themselves, for example, crystalline photovoltaic devices composed of single-crystal or polycrystalline silicon and thin film photovoltaic devices using compound semiconductors, have been intensively studied and developed. Relevant technologies other than the photovoltaic elements also have been studied and developed. Examples of such technologies include technologies regarding effective conversion of the incident light to electrical energy and effective extraction of the generated electrical energy to the exterior.
U.S. Pat. No. 4,419,533 discloses a technology for effectively converting the incident light to electrical energy, in which a metal reflection layer has unevenness and a barrier layer composed of zinc oxide, etc., is provided to prevent diffusion of the constituent elements of the reflection layer to the photovoltaic element. U.S. Pat. No. 5,500,055 discloses another technology for trapping the incident light in the device, in which a smooth reflection layer is provided, and the surface of the barrier layer is etched by an aqueous solution to form unevenness on the surface.
A technology for effectively extracting the generated electrical energy is disclosed in U.S. Pat. No. 4,532,372, wherein a transparent conductive layer is provided to prevent self-consumption of the generated electrical energy caused by short-circuiting due to photovoltaic device defects.
Fabrication processes are also disclosed. For example, Japanese Patent Laid-Open No. 62-54921 discloses a process for achieving an increased photovoltaic current flow, in which a substrate is exposed to hydrogen plasma before fabricating an amorphous-silicon semiconductor device. In Japanese Patent Laid-Open No. 8-217443, zinc oxide is produced from an aqueous solution containing a nitrate and zinc by electrodeposition.
The present inventors have reviewed the configuration of the transparent conductive layer disclosed in U.S. Pat. Nos. 4,419,533 and 5,500,055. In such a configuration, a reflection layer is formed and a transparent conductive layer is provided thereon. The transparent conductive layer functions as a barrier layer for preventing diffusion of the constituent material of the reflection layer to the photovoltaic element, scatters the incident light to effectively use it, and prevents short-circuiting by defects in the semiconductor layer.
Therefore, optimized sputtering and electrodeposition conditions enable the reliable formation of an uneven transparent conductive layer. Further, etching of the uneven surface with an aqueous solution further enhances the surface roughness. In the operation under a load, however, the voltage sometimes drops. In such a case, the photovoltaic device has a fill factor far from 100%; hence, there is room for improvement of the device in this regard.
The present inventors have further studied the surface treatment with hydrogen plasma and then oxygen plasma as disclosed in Japanese Patent Laid-Open No. 62-54192. When an oxide transparent conductive layer is subjected to such treatment, short-circuiting often occurs and thus a photovoltaic device having stable characteristics is not obtainable. According to electron microscopy of the transparent conductive layer, some cracks and sharp ridges are observed on the surface of the deposited film. It is presumed that reduction by hydrogen and oxidation by oxygen cause changes in the composition in the film, and a stress is applied onto the surface.